Module 3 Lesson 2 - Enthalpy Changes in Chemistry
Objectives Must Recall and use the term STANDARD CONDITIONS Should Define the terms standard enthalpy change of REACTION, FORMATION AND COMBUSTION. Could Calculate and write chemical equations for the standard enthalpy changes
Starter - AfL When a chemical reaction takes place heat may be given out or taken in. AfL - Sketch an energy profile for an exothermic reaction and an endothermic reaction showing activation energy AND the overall energy change.
Enthalpy of reaction The overall energy change is referred to as the enthalpy of reaction If the conditions are standard conditions then we call the energy change the Standard enthalpy of reaction, ΔHr Φ
Main: What are standard conditions? LEARN – quoting these is worth marks on the exam… Standard conditions are a pressure of 100kPa, 298K (25°C) and a concentration of 1.0 M (for reactions with aqueous solutions). Standard stateis the physical state of a substance under standard conditions eg magnesium has the standard state Mg (s), hydrogen has the standard state H2(g) and water has the standard state H2O (l)
Definitions – part 1 Φ The standard enthalpy change of reaction ΔHr is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states. H2(g) + ½ O2 (g) H2O(l) ΔHr = -286kJmol-1 AfL – is this exo or endothermic? Φ
AfL Quiz – Part 1 ΔHr Two enthalpy changes of reaction are shown below: N2(g) + 3H2(g) 2NH3 (g) ΔH = -92 kJmol-1 N2O4 (g) 2NO2 (g) ΔH = +58 kJmol-1 What is the enthalpy change of reaction for: ½N2(g) + 1½ H2(g) NH3(g) ΔH = _____ kJmol-1 ½N2O4(g) NO2(g) ΔH = ______ kJmol-1
Definitions – part 2 Φ The standard enthalpy change of combustion ΔHc is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. C2H6(g) + 3½ O2 (g) 2CO2(g) + 3H2O(l) ΔHr = -1560 kJmol-1 AfL – is this exo or endothermic? Φ
AfL Quiz part 2- ΔHc Write equations for the change that occurs during the enthalpy change of combustion for each of the following compounds • CH4(g) • C3H8(g) • CS2(l) • CH3OH(l) • C2H5OH(l)
Definitions – part 3 Φ The standard enthalpy change of formation ΔHf is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. H2(g) + ½ O2 (g) H2O(l) ΔHf= -286kJmol-1 ΔHf = -286 kJmol-1 AfL – what else could we refer to this as? Φ Φ
AfL Quiz part 3- ΔHf Write equations for the change that occurs during the enthalpy change of formation for each of the following compounds. • C2H4(g) • C2H6(g) • C2H5OH(l) • CaO(s) • Al2O3(s)
Quick tips Remember that: • for ΔHr you are using the moles given in the equation • for ΔHc you are burning 1 mole of fuel with however much oxygen it takes • for ΔHf you are forming 1 mole of compound from however much of its elements are required • ΔHf for an element is 0 kJmol-1
Module 3 Lesson 3 – bond enthalpies
Quick quiz – back page of your book • Reactions where the products are at a lower energy than the reactants are endothermic (TRUE/FALSE) • Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE) • A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE) • How can we tell if a reaction is exothermic or endothermic? • Sketch the energy profile for an endothermic reaction. • When methane (CH4) burns in oxygen (O2) bonds between which atoms need to be broken? FALSE FALSE TRUE Measure the temperature change C—H bonds and O=O bonds
Objectives Must Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form Should Identify which bonds are broken and which are made when a chemical reaction takes place. Define and use the term average bond enthalpy. Could Use data on the energy needed to break covalent bonds to estimate the overall enthalpy change of reaction.
Activation energy revisited • What is the activation energy of a reaction? • The energy needed to start a reaction. • BUT what is that energy used for and why does the reaction need it if energy is given out overall? • The activation energy is used to break bonds so that the reaction can take place.
Burning methane Consider the example of burning methane gas. CH4 + 2O2 CO2 + 2H2O This reaction is highly exothermic, it is the reaction that gives us the Bunsen flame. However mixing air (oxygen) with methane is not enough. I need to add energy (a flame).
What happens when the reaction gets the activation energy? C H H H H O O O O Bond Breaking Bond Forming H O Energy in chemicals O C O O O H H H H H O C O O H H Progress of reaction
Using bond enthalpies By using the energy that it takes to break/make a particular bond we can work out the overall enthalpy/energy change for the reaction. Sum (bonds broken) – Sum (bonds made) = Energy change ΔH = (bond enthalpies of bonds broken) - (bond enthalpies of bonds made)
Exo or endo (again) • If the overall energy change is negative is this an exothermic or endothermic reaction?
Copy and complete _______ ______is the energy needed to start the reaction by __________ chemical _____ in the reactants. Energy is given out when ______ chemical bonds form. The overall energy change for a reaction can be worked out as the energy required for all bonds _______ minus the energy gained by ______ bonds. [Activation energy, new, bonds, broken, breaking, making]
Copy this table of bond enthalpies into your notes Definition Average bond enthalpy is the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species.
Can you work out the energy change for this reaction? CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) Tips: • Draw the reactants and products and work out the bonds you are breaking and the ones you are making. • In an equation to represent ΔHc you must not put a balancing number in front of the substance being burnt. If you do, then more than 1 mol would have been combusted.
Challenge question • The true value for the energy change is often slightly different from the value calculated using bond enthalpies. • Why do you think this is?
Exam paper questions • Attempt the following questions (OCR GCSE C7 questions).
Summary Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form Identify which bonds are broken and which are made when a chemical reaction takes place. Use data on the energy needed to break covalent bonds to estimate the overall energy change for a reaction.
Module 3 Lesson 4 – Calorimetryand enthalpy change of combustion
Objectives Must Follow a set of instructions to set up practical equipment safely and quickly Should Use the equipment provided to investigate the heat given out when a substance burns Could Calulate a value for ΔHc.
Equation for ΔHc Balance the following equations for burning methanol and ethanol in air and add the correct state symbols for the substances in their standard states: __CH3OH + __O2 __CO2 + __H2O __C2H5OH + __O2 __CO2 + __H2O In an equation to represent ΔHc you must not put a balancing number in front of the substance being burnt. If you do, then more than 1 mol would have been combusted. AfL – what are standard conditions?
The correct equations CH3OH(l) + 1½O2(g) CO2(g) + 2H2O(l) C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)
How does calorimetry work? • A known volume of cold water is measured into the beaker/can • The starting temperature of the water is recorded • The water is heated using the flame from the burning fuel • The final temperature of the water is recorded • The spirit burner containing the fuel is weighed before and after the experiment.
Practical • Follow the instructions on Activity sheet 26 to compare the enthalpy of combustion of methanol and ethanol.
Calculations The amount of energy transferred from the burning fuel to the water (the surroundings) in the calorimeter can be calculated if you know: • the mass of water heated • the temperature rise AfL – can you remember the equation required?
Calculations ΔE = mcΔT Energy transferred (joules, J) = mass of water heated × 4.2 × temperature rise Use this equation to calculate the energy transferred for each of your experiments.
Energy per gram of fuel • You can compare fuels by measuring the mass of fuel burned in the experiment. The best fuel is likely to release the most energy per gram of fuel. This is worked out using: Energy released (J/g of fuel) = Energy transferred to water (J) ÷ Mass of fuel burned (g)
Errors The figure you have calculated is lower than the figure found using bond energies (remember breaking and making bonds). Why do you think this could be?